With recent rapid progress of information transmitting systems, silver halide photographic materials have been increasingly required to have high sensitivity. This high sensitivity is a necessity as well in the field of negative working as of positive working silver halide photographic materials. A negative working photographic material is one giving a negative image while a positive working photographic material is one giving a positive image. The term negative image means that image reversal occurred while the term positive image indicates that no image reversal took place.
So, for a photographic material to be suitable for use in graphical applications it is required that the material is of high speed to the applied illumination, it being a classical analogous one or a so-called high intensity-short time exposure (flash exposure or scanning exposure) and yields images of high contrast and high resolving power.
Photographic materials for graphic applications include photographic films and papers used after processing as interoriginals in a process for preparing a lithographic printing plate and silver salt diffusion transfer based (hereinafter called DTR-) lithographic printing plates disclosed in e.g. U.S. Pat. No. 4,501,811 and U.S. Pat. No. 4,784,933. With the latter materials a lithographic printing plate can be obtained without the need of a photographic interoriginal.
In most cases it is required that said interoriginals have a negative image because most printing plates are negative working. Time and cost are then saved when said interoriginals are prepared by phototypesetting when using a positive working silver halide photographic material e.g. a photographic material comprising a positive working (hereinafter called direct-positive) silver halide emulsion.
In the conventional DTR-process i.e. a DTR-process wherein a photographic material comprising a negative working (hereinafter called negative) silver halide emulsion is used a positive image is formed in the receiving layer.
At the exposed areas of the silver halide emulsion layer the silver halide is developed (chemical development) and thus cannot be dissolved anymore to diffuse to the receiving layer.
At the unexposed areas of the silver halide emulsion layer the silver halide is converted into a soluble silver complex salt and is transferred to the receiving layer, where it forms a silver image usually in the presence of physical development nuclei.
If a negative image of the original is to be formed in the receiving layer a photographic material comprising a direct-positive silver halide emulsion should be used. The formation of negative images from the original are interesting where positive DTR-images are desired from negatives or vice versa.
It is further known to make lithographic printing plates by the DTR-process.
Here too printing plates made by the conventional DTR-process are of the positive type and a photographic material comprising a direct-positive silver halide emulsion is required in the DTR-process to obtain a positive printing plate starting from negative originals.
Furthermore the conventional positive type DTR-process for direct plate making by lasers and scanners e.g. computer to plate applications which are widely employed, have the disadvantage of requiring a long exposure time since the non-image areas of the negative silver halide emulsion layer should be exposed in order to obtain in the image receiving layer image areas that are printing. Thus for computer to plate applications it is also desirable to have a DTR-material comprising a direct-positive silver halide emulsion.
For the formation of a direct-positive image in the silver halide emulsion layer two main types of direct positive silver halide emulsions are known.
One type is a silver halide emulsion that has been prefogged. Such types of emulsion are commonly known as Herschel reversal emulsions and are described in e.g. U.S. Pat. No. 3,367,778. U.S. Pat. No. 3,733,199 discloses the use of such a type of emulsion for use in a diffusion transfer process. U.S. Pat. No. 4,149,889, U.S. Pat. No. 4,175,965 and JP-Pi 01-96648 describe the use of Herschel reversal emulsions for the production of a negative type lithographic printing plate.
The second type of direct positive emulsions are inner latent image type silver halide emulsions that have not been previously prefogged. Such a type of direct positive emulsions is disclosed in e.g. EP-A365.926. U.S. Pat. No. 4,309,499 discloses its use in a DTR imaging element.
This last type of direct positive emulsion shows normally a higher speed than the first type but can not be developed in the classical way. While with the first type of direct positive type emulsions a positive image in the emulsion layer is obtained by effecting a normal surface development, the second type of direct positive type emulsions requires a supplementary fogging treatment subsequent to the image-wise exposure before or while applying a normal surface development.